Amorphous particles of synthetic SiO2 are, e.g., obtained by flame hydrolysis or oxidation of silicon compounds, by polycondensation of organic silicon compounds according to the so-called sol-gel method, or by hydrolysis and precipitation of inorganic silicon compounds in a liquid. In the industrial production of synthetic quartz glass, such SiO2 primary particles are also obtained as so-called soot or filter dust.
SiO2 primary particles are finely divided. In the sol-gel method, one typically obtains mean particle sizes in the range of 0.5 μm and 5 μm, and of less than 0.2 μm in flame hydrolysis. They are inter alia used as starting material for the production of quartz glass. On account of their small particle size and the accompanying large specific surface area, the SiO2 primary particles are, however, not free-flowing, extremely sinter-active, and can be easily blown away, which impedes immediate fusion into quartz glass. Therefore, they are normally granulated and pre-densified in advance. Examples of suitable build-up or press granulation methods are roll granulation in a pan granulator, spray granulation, centrifugal atomization, fluidized-bed granulation, granulation methods using a granulating mill, compaction, roller presses, briquetting, flake production, or extrusion.
During granulation, discrete, rather large agglomerates are formed due to agglomeration of the SiO2 primary particles; these are herein called “SiO2 granulate particles” or briefly “granulate particles.” They have a multitude of open pores, forming a large pore space. In their entirety, the granulate particles form a “porous SiO2 granulate,” i.e., “open-pore SiO2 granulate.”
Such porous SiO2 granulate is free-flowing and, by comparison with finely divided SiO2 dust, shows an increased bulk weight that can be further increased by thermal or mechanical treatment.
Porous SiO2 granulate is used directly as a filler or for producing opaque quartz glass. A method for producing synthetic opaque quartz glass by melting SiO2 granulate is known from DE 199 62 451 A1. The starting material is formed from amorphous, nanoscale, pyrogenic SiO2 particles which are produced by flame hydrolysis of SiCl4. An aqueous dispersion is produced therefrom and moisture is removed from the dispersion under continuous stirring in a mixer until the dispersion decomposes, forming a crumbly mass. The granulate particles produced thereby are roundish and have diameters in the range between 160 μm and 1000 μm and a specific (BET) surface area of 50 m2/g. The open-pore granulate is thermally pre-densified in a continuous furnace with a throughput of 10 kg/h at a temperature of about 1200° C. in a chlorine-containing atmosphere. Due to this thermal pretreatment, a temperature gradient is evolving over the volume of the individual granulate particles, resulting in low density in the central region and in high density in an outer region. Moreover, the granulate is cleaned during this treatment and freed of hydroxyl groups. The mean particle size of the pre-densified SiO2 granulate is 420 μm and has a specific (BET) surface area of 34 m2/g and a tamped density of 1.1 g/cm3. The total content of the impurities of Li, Na, K, Mg, Ca, Fe, Cu, Cr, Mn, Ti, and Zr is less than 200 wt. ppb.
The synthetic SiO2 granulate which continues to be open-pored is used for producing a tubular component of opaque quartz glass. A layer-shaped bulk material of the granulate is fused zone by zone on the inner wall of a tubular metal mold which is rotating about its longitudinal axis by an electric arc, starting from the inner bore. This results in the formation of a vitrification front progressing to the outside toward the metal mold, the front forming the boundary between the still open-pore layer region and an already partly molten region. It is true that most of the pore space of the granulate is closed in this process by sintering and collapsing, but the entrapped gases lead to the formation of a multitude of bubbles in the quartz glass on which incident light is scattered; this leads to opacity both in the visible spectral range and particularly also in the infrared spectral range.
However, in comparison with the production of opaque quartz glass, the fusion of the open-pore SiO2 granulate into transparent synthetic quartz glass poses problems. On the one hand, this is due to the fact that gas-filled pores are enclosed during fusion of the porous SiO2 granulate and may lead to bubbles which cannot be removed from the highly viscous quartz glass mass or can only be removed at a very slow pace. On the other hand, the open-pore SiO2 granulate has a strong heat-insulating effect, making it difficult to heat the bulk granulate to a uniformly high temperature. The excellent heat-insulating effect is, e.g., demonstrated in that the pressed pyrogenic SiO2 particles are used for thermal insulation in high-performance insulation panels, such as vacuum insulation panels.
Therefore, an adequate heat energy for melting a bulk material of porous SiO2 granulate or for melting a molded body of the granulate can be supplied from the outside only at a very slow pace and under great energy expenditure. This problem increases with the volume of the bulk material or the molded body, respectively, and may lead to an irregular temperature distribution during fusion and thus to inhomogeneous properties of the vitrified component.
Accordingly, for sophisticated applications which require the absence of bubbles and uniform material properties of the end product, a previous thermal densification is considered to be necessary, ideally until complete vitrification of the porous granulate occurs. The dense glass particles obtained by complete vitrification of the porous SiO2 granulate particles are also called “quartz glass particles” here and in the following, the quartz glass particles in their entirety forming synthetic “quartz glass granules.” Many different techniques are known for the production of dense quartz glass granules from porous SiO2 granulate.
It is, e.g., suggested in EP 1 076 043 A2 that porous SiO2 granulate should be poured into a combustion gas flame to finely distribute it therein and to vitrify the same at temperatures of 2000-2500° C. The granulate is preferably obtained by spray granulation or wet granulation of filter dust and has grain sizes in the range of 5-300 μm. Prior to vitrification it can be heated and pre-densified by treatment with microwave radiation.
The degree of sintering of a given granulate particle depends on its particle size and on the heat input which, in turn, is defined by the residence time in the combustion gas flame and by the flame temperature. As a rule, however, the granulate has a certain particle size distribution, and the combustion gas flame has regions of different flow velocities and flame temperatures. This leads to irregular sintering degrees that can hardly be reproduced. Moreover, there is the risk that the quartz glass particles are contaminated by the combustion gases. Specifically, loading with hydroxyl groups using a hydrogen-containing combustion gas flame should here be mentioned; such loading leads to a frequently undesired reduction of the viscosity of the quartz glass.
EP 1 088 789 A2 suggests that for the vitrification of porous SiO2 granulate, the granulate should first be purified by heating in an HCl-containing atmosphere in a rotary furnace, subsequently calcined in a fluidized bed, and then vitrified in a vertical fluidized bed apparatus or in a crucible in vacuum, helium or hydrogen into synthetic quartz glass granules.
In a similar method according to JP 10278416 A, synthetically produced particulate SiO2 gel is continuously densified in a rotary furnace. The rotary tube is divided into several temperature zones covering the temperature range of 50° C. to 1,100° C. The particulate SiO2 gel with particle sizes between 100 μm and 500 μm is first freed of organic constituents in the rotary tube, which is rotating at 8 rpm, by supply of an oxygen-containing gas. In a sintering zone in which the furnace atmosphere contains oxygen and optionally argon, nitrogen or helium, it is sintered into open-pore SiO2 granulate. The sintered SiO2 granulate still contains a high concentration of silanol groups. To eliminate these groups and to achieve complete densification, the sintered but still open-pore granulate is calcined in the end at an elevated temperature of 1300° C. in a quartz glass crucible with an inner diameter of 550 nm in batches of 130 kg and vitrified.
DE 10 2012 006 914 A1 discloses a method for producing synthetic quartz glass granules by vitrifying a free-flowing SiO2 granulate. This method comprises the steps of: granulating pyrogenically produced silicic acid to form an SiO2 granulate of porous granulate particles, drying the SiO2 granulate, cleaning the SiO2 granulate by heating in a halogen-containing atmosphere, and vitrifying the cleaned SiO2 granulate in a treatment gas which contains at least 30% by volume of helium and/or hydrogen to form vitrified quartz glass granules. Cleaning and vitrification of the SiO2 granulate are each carried out in a rotary furnace which comprises a rotary tube made of a ceramic material.
WO 88/03914 A1 also teaches about the reduction of the BET surface area of an amorphous porous SiO2 powder by using a rotary furnace in a helium- and/or hydrogen-containing atmosphere. It is suggested that SiO2 soot dust should be mixed with water, resulting in a moist crumbly mass. This mass is put into a rotary furnace and densified at a temperature of 600° C. into a powder having grain sizes of 0.1 mm to 3 mm. The pre-densified SiO2 powder is subsequently vitrified in a separate furnace.
DE 10 2004 038 602 B3 discloses a method for producing electrically molten synthetic quartz glass for use in lamp and semiconductor manufacture. Thermally densified SiO2 granulate is used as a starting material for the electrically molten quartz glass. The granulate is formed by granulating an aqueous suspension consisting of amorphous, nanoscale and pyrogenic SiO2 particles which are produced by flame hydrolysis of SiCl4. Roundish granulate grains with outer diameters in the range between 160 μm and 1000 μm are obtained. The granulate is dried in a rotary furnace at about 400° C. and densified at a temperature of about 1420° C. up to a BET surface area of about 3 m2/g. For the complete vitrification, the individual grains of the granulate are subsequently heated under different atmospheres, such as helium, hydrogen or vacuum. The heating profile during vitrification of the granulates involves heating up to 1,400° C. at a heating rate of 5° C./min and a holding time of 120 min. After this treatment, the individual granulate grains are vitrified in themselves. The grains are present individually without being fused into a mass. The granulate is further processed in an electric melting process into quartz glass; for instance, it is fused in a crucible into a molded body or is continuously pulled in a crucible pulling method into a strand.
WO 2007/085511 A1 describes a granulation method in which finely divided SiO2 start powder is agglomerated mechanically, also using lubricants or binders, by way of roller compaction into rather coarse particles and is densified by mechanical pressure. The SiO2 start powder is passed between and through oppositely rotating profiled rollers and is thereby densified into SiO2 granulate which is obtained in the form of so-called “crusts.” These crusts or fragments thereof are dried at a temperature in the range of 400° C. to 1100° C. in a halogen-containing atmosphere and are densely sintered in the range of 1200° C. to 1700° C. into quartz glass granules.
These quartz glass granules can be directly fused by flame or plasma burners or in electrically heated melting crucibles or melting molds and processed into components of transparent or opaque synthetic quartz glass, such as tubes, rods, plates, holders, bell jars, reactors, casting channels, flanges or crucibles for semiconductor or lamp manufacture and chemical process engineering (this process step is also called “direct fusion” in the following).
Alternatively, in a process variant to be called “indirect fusion,” a porous molded body is first produced from the dense quartz glass granules in ceramic-mechanical molding steps and the molded body is sintered into the quartz glass component. Such a method is, e.g., known from U.S. Pat. No. 4,042,361 A. The production of a quartz glass crucible with the help of a slip casting method using synthetic quartz glass granules is described therein. The quartz glass granules are produced from pyrogenically produced SiO2 powder, which is obtained as filter dust in the flame hydrolysis of a silicon compound, in that a gel is produced from the loose SiO2 powder first by mixing into water and stirring, the solid content of said gel varying between 30% by wt. and 45% by wt. depending on the type and speed of the stirring process. The fragments obtained after drying of the gel are sintered at temperatures between 1150° C. and 1500° C. into dense quartz glass granules. These are subsequently finely milled into grain sizes between 1 μm to 10 μm and stirred into an aqueous slip. The slip is cast into a crucible mold, and the layer adhering to the edge of the crucible is dried to form a porous green body. The green body is then vitrified at a temperature between 1800° C. and 1900° C. into the quartz glass crucible.
The pre-vitrification of the open-pore SiO2 granulate into dense quartz glass granules that are as bubble-free as possible constitutes, in principle, an appropriate intermediate step for the fusion of the granulate into low-bubble transparent quartz glass. A separate vitrification process with temperatures above the softening temperature of quartz glass, i.e., above 1150° C., typically about 1400° C., must however be accepted in return.
Irrespective of this, it is not trivial and is often also not possible to produce bubble-free quartz glass granules from porous SiO2 granulate. The best results are achieved during vitrification under vacuum or in helium or hydrogen as a sintering aid for improving the heat transfer or for minimizing bubbles. This, however, increases not only the consumption costs, but also the risk of safety (in the case of hydrogen because the risk of explosion during reaction with oxygen). This vitrification process is long-winded and entails high energy consumption.
However, even the direct or indirect fusion or the sintering of a molded body from completely dense quartz glass granules does not readily eliminate the problem of bubble formation in the resulting quartz glass, for the space between the dense quartz glass particles may contain gases that during the fusion process are entrapped within the viscous quartz glass melt and can hardly escape or be removed by homogenization measures. They cause bubbles and other disorders in the quartz glass.
Therefore, degasification measures are indispensable for avoiding bubbles not only during vitrification of the porous SiO2 granulate into dense quartz glass granules, but also in the direct or indirect fusion process whenever one aims at a bubble-free transparent quartz glass. Suitable measures include the fusion or the sintering of the quartz glass granules under vacuum or in an atmosphere of helium or hydrogen, but with the above-explained efforts in terms of time, energy, and material.